Yup. It's a gimmick. Just find a way to get them on their backs, and the ball-turning is just an ordinary reaction - The way we turn it into an "act" is an illusion. It almost seems a little cruel.
The flies act just like those I used to put on a CO2 supplied platform in genetics class in college. The CO2 knocks them silly and their little legs move the same. No one thought to put painted ping-pong balls on their feet though. Darn.
It almost seems a little cruel.